1. Field of the Invention
The invention relates to a method for manufacturing a micromechanical structure, and to a micromechanical structure.
2. Description of the Related Art
Although it is applicable to any micromechanical components, the present invention and its underlying problem will be explained with reference to acceleration and rotation rate sensors.
Published German patent application document DE 195 37 814 A1 discloses a method for manufacturing micromechanical sensors, for example acceleration and rotation rate sensors. Movable silicon structures, whose motions are sensed quantitatively by determining capacitance changes, are generated. The movable silicon structures are generated in an etching step, trenches being generated with a high aspect ratio in the silicon layer. In a second step a sacrificial layer, for example an oxide layer, is removed from beneath the micromechanical functional layer made of silicon. In a subsequent process the movable silicon structures thereby obtained are hermetically closed off, for example with a cap wafer that is applied using a seal-glass soldering process.
Published German patent application document DE 199 61 578 A1 discloses the generation, directly on a base wafer by way of further process steps, of a cap over the movable sensor surfaces. For this, a layer of oxide is applied on the base wafer, on which the movable silicon structures have already been etched but the sacrificial layer has not yet been removed. The thickness of this oxide layer is small compared with the thickness of the micromechanical functional layer. The oxide layer must, however, close off the trenches, and for this reason it is possible to provide only narrow trenches. A layer that is transparent to the medium used for sacrificial layer etching, for example a thick polysilicon layer into which very narrow holes are etched, is applied onto the oxide layer. The cover layer having thereby been made transparent, all sacrificial layers located therebeneath are then removed in order to make the silicon structures movable. In a final step, the cover layer is hermetically sealed with a further oxide layer or metal layer or polysilicon layer. Only movable structures having relatively little freedom of movement can be arranged beneath the cover layer.
A situation similar to that relating to the closing off of deep, broader structures also exists in the region of the actual movable structures. If a movable structure is arranged over a buried conductive path, an edge in the movable structure also occurs, because of the manufacturing method, at edges in the buried conductor path. This structure can strike against the edge of the buried conductor path and damage it. To prevent this, bumps can be generated in the movable structure by way of an additional sacrificial layer. The movable structure then, with its bumps, strikes against the conductor areally at defined positions, but freedom of movement is thereby further limited. This concept functions, however, only if the buried conductor path is thinner than the sacrificial layer.
This situation also exists when two movable structures are to be disposed one above another, or when a conductor path plane is to be used over a movable structure. In such cases it is usually necessary to design the movable structures to be thicker than the sacrificial layer.